Lots of good feedback so far. My responses, in no
particular order ...
1. Measuring grid current: thought of that, and felt
that I could use some kind of low-drop shunt to
directly measure at the bottle ... would have great RF
characteristics and also super accurate DC specs, not
hard to fabricate, either
2. Peril of low fil resistance at cold power-on:
actually, I leave some juice running on the fils at
all times. The fils are run at a very low current
when "off" which allows the minimum load necessary to
keep the power supply happy, and never allows the fils
to go to totally "ice" state. There are two relays:
one is an SPST that shorts out a series resistor when
powering up the amp. Besides, the power supply
current-limiter is easily adjusted to flatten out the
peak, anyway. Much easier to do this with a DC supply
having all the bells and whistles than to have to
design either a ramp-start xfmr primary gizmo or a
nicer (like the L7 fil xfmr) secondary, which is
perfect in that it naturally limits inrush; if it was
only available anymore. The second relay is a big
DPDT 100A teclco-surplus latching contactor that
allows me to reverse the polarity of the fil voltage
every 100 hours (I'll have a Hobbs meter on the PS
deck).
3. Very easy to adjust the switcher to make anywhere
from 4.5V to 6.5 v with its trimpot or remote control,
plus it has a remote sense line that can accomodate
the small I2R loss of the 8 AWG and Anderson PowerPole
jumper I am planning. Actual fil voltage will be
metered in my amp, with a dedicated meter. I'd have
to buy a $$ Variac to try the same stunt with a
conventional design.
4. The 100W supply will loaf at the 75W fil
requirement of a 3-500z, and if I upgrade to the
3CX1500D7, I can simply put two of these in parallel
as they are designed to do that (diode-sum the
outputs). $2 for a 100W fil power supply makes the
whole thing seem cheaper.
I am glad I haven't heard any showstoppers yet (not
even from Rich!). And, the savings here are adding
up, so far, well into a three-figure number, not
counting the hernia I am not going to have.
Now, if I only had a real metal shop setup to build
the cabinets ...
Boy, I need to get a job soon. (!)
Steve KZ1X/4
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